Patent Grant
7944459
1. Technical Field
The present invention relates to a technique that controls the light amount of a light-emitting element, such as an organic light-emitting diode (hereinafter, referred to as ‘OLED’).
2. Related Art
A light-emitting device having a plurality of light-emitting elements arranged therein is used as an exposure head that exposes a photosensitive member so as to form a latent image or a display device that displays various images. The characteristics of such a light-emitting element are being degraded according to a degree of light-emission in the past (for example, the number of times of light-emission) as time passes. In the light-emitting device, the degree of light-emission of each of the light-emitting elements varies according to the shape of an image or a gray-scale level, and thus a variation in characteristic of each light-emitting element (for example, light-emission efficiency) occurs. In particular, when a plurality of images having a common shape are successively output (for example, in an image forming apparatus using a light-emitting device as an exposure head, when the same image is printed in quantity), the variation in characteristic of each light-emitting element is expanding as time passes.
In order to solve the variation in characteristic due to a time-variant degradation of each light-emitting element, for example, JP-A-2003-334990 or JP-A-2002-361924 discloses a technique that causes each light-emitting element to emit light additionally according to the number of times of light-emission of each light-emitting element in the past. According to this technique, the sum of the number of times of light-emission is made uniform, for a plurality of light-emitting elements, and thus the variation in characteristic of each light-emitting element or luminance irregularity can be suppressed.
However, in the technique disclosed in JP-A-2003-334990 or JP-A-2002-361924, in order to hold the number of times of light-emission in each of the plurality of light-emitting elements in the past, a mass storage device is required. Accordingly, the circuit size of the light-emitting device becomes large, and manufacturing costs are increased when the total number of the light-emitting elements or the number of gray-scale levels for a high-definition image, the number of data (the number of times of light-emission) or the number of digits stored in the storage device needs to be increased, and thus the above problem becomes more critical.
An advantage of some aspects of the invention is that it reduces a storage capacity required for suppressing a variation in characteristic of each light-emitting element.
According to a first aspect of the invention, a light-emitting device includes a plurality of light-emitting elements, each emitting light with a light amount according to a driving signal, a storage unit (for example, a storage unit 44 of
According to the first aspect of the invention, the second gray-scale data is generated from the first gray-scale data of each light-emitting element such that the larger the gray-scale value of the first gray-scale data is, the smaller the gray-scale value of the second gray-scale data is. Further, each light-emitting element is driven on the basis of the first gray-scale data in the first period, and is driven on the basis of the second gray-scale data in the second period. According to this configuration, a degree of light-emission of each light-emitting element is made uniform over the plurality of light-emitting elements, as compared with a case where each light-emitting element is driven on the basis of only the first gray-scale data. Therefore, a variation in light amount of each light-emitting element due to a time-variant degradation can be suppressed. Besides, according to the first aspect of the invention, the degrees of light-emission by the first gray-scale data and the second gray-scale data are not necessarily completely matched with each other for each light-emitting element.
Moreover, the light-emitting elements herein are parts for radiating light. More specifically, the light-emitting elements are elements that emit light upon application of electrical energy. The specific structure or material of the light-emitting element herein is arbitrarily selected. For example, an element having electrodes and a light-emitting layer formed of an organic EL material or an inorganic EL material interposed between the electrodes can be used as the light-emitting element of the invention. In addition, various light-emitting elements, such as an LED (Light Emitting Diode) element, an element that emits light by plasma discharge, and so on can be used in the invention Further, the driving signal is specified by, for example, a level (current value or voltage value) and a pulse width (that is, the driving signal has a level component and a pulse width component). ‘The light amount according to driving signal’ herein is a light amount according to the level of the driving signal or a light amount according to the pulse width of the driving signal.
In the invention, ‘such that the larger the gray-scale value assigned by the first gray-scale data is, the smaller the gray-scale value assigned by the second gray-scale data becomes’ means that, paying attention to specified gray-scale values g1a and g1b among all the gray-scale values to be assigned by the first gray-scale data (however, g1a<g1b), a gray-scale value g2a of the second gray-scale data generated from the first gray-scale data having a gray-scale value g1a is larger than a gray-scale value g2b of the second gray-scale data generated from the first gray-scale data having a gray-scale value g1b (g2a>g2b). As regards all the gray-scale values assigned by the first gray-scale data and all the gray-scale values assigned by the second gray-scale data generated by the first-gray scale data, the same relationship is not necessarily established. For example, as described above, when g1b is larger than g1a (‘g1a<g1b’), if the relationship ‘g2a>g2b’ is established, it still falls within the scope of the invention, regardless of the relationship between a certain gray-scale value g1c assigned by the first gray-scale data (≠g1a and g1b) and a gray-scale value g2c of the second gray-scale data generated on the basis of the gray-scale value g1c.
According to a specific aspect of the invention, the first period (for example, a first period P1 of
According to a specific aspect of the invention, the second period may be shorter than the first period. According to this configuration, as compared with the configuration that the first period and the second period have the same time length, a time length that can be originally used to form an image in the first period can be secured relatively long. Therefore, the image can be efficiently formed.
In the above aspects, a specific configuration for making the second period shorter than the first period is arbitrarily selected. For example, the total number of the light-emitting elements that actually emit light in the second period may be made smaller than the total number of the light-emitting elements that emit light in the first period, and thus the second period may have a time length shorter than the first period. However, according to a preferred aspect of the invention, the driving signal supplied to each light-emitting element may become a level (current value or voltage value) for causing the light-emitting element to emit light by a pulse width according to the first gray-scale data of a first unit period (for example, a unit period U1 of
However, if the level of the driving signal is set to the first period and the second period, and the second unit period is set to have a time length shorter than the first period, there is a possibility that degrees of light-emission of each light-emitting element in the first period and the second period are different from each other. According to a preferred aspect of the invention, the driving signal supplied to each light-emitting element may become a level (for example, an on current value Ia of
By the way, among the light-emitting elements, there may be an element that has different states of the time-variant change in characteristic when the level of the driving signal is fixed and the pulse width is changed, and when the pulse width of the driving signal is fixed and the level is changed. In the light-emitting device that uses such a light-emitting element, the pulse width and the level of the driving signal according to the second gray-scale data are set such that the state of the time-variant change in characteristic of the light-emitting element when the driving signal according to the second gray-scale data assigning a predetermined gray-scale value may be supplied approximately matches with the state of the time-variant change in characteristic of the light-emitting element when the driving signal according to the first gray-scale data assigning the predetermined gray-scale value is supplied. According to this configuration, a speed of time-variant characteristic degradation of each light-emitting element can be made uniform for the plurality of light-emitting elements.
Moreover, ‘the state of the time-variant change in characteristic of the light-emitting element’ means the relationship between a time elapsed from a time point at which the light-emitting element is produced (or a time elapsed from a time point at which the use of the light-emitting device starts) and the characteristic of the light-emitting element. In general, it is a characteristic change speed of the light-emitting element. Further, lifespan representing a time until a characteristic value (for example, a light amount when a predetermined gray-scale level is assigned) of the light-emitting element is lowered to a predetermined value corresponds to the state of the change in characteristic of the light-emitting element in the invention. The characteristic of the light-emitting element includes, for example, a light amount of the light-emitting element when a predetermined gray-scale value is assigned or a relative ratio (light-emission efficiency) between the value of a current supplied to the light-emitting element and a light amount at that time.
For example, a time-variant lowering speed of the light amount of each light-emitting element, such as an OLED element, may be approximately in proportion to the pulse width of the driving signal and to an M power (where M is a real number) of the level of the driving signal. In the configuration that uses such a light-emitting element, the light-emitting element having a predetermined gray-scale value assigned by the first gray-scale data emits light with a light amount La upon supply of a driving signal having a pulse width Wa, the level of the driving signal according to the second gray-scale data may be determined such that a light amount Lb of the light-emitting element to which a driving signal having a pulse width Wa/u (where u>1) according to the second gray-scale data assigning the predetermined gray-scale value satisfies the equation Lb/La=u1/M. Alternatively, when the light-emitting element having a predetermined gray-scale value assigned by the first gray-scale data emits light with a light amount La upon supply of a driving signal having a pulse width Wa, a pulse width Wb of a driving signal that has a level determined so as to cause the light-emitting element to emit light with a light amount La×v (where v>1) according to the second gray-scale data assigning the predetermined gray-scale value may satisfy the equation Wb/Wa=v−M.
The light-emitting device according to the aspect of the invention is used in various electronic apparatuses. As the electronic apparatus, there is an image forming apparatus that has the light-emitting device according to the aspect of the invention as an exposure device (an exposure head). The image forming apparatus includes an image carrier having an image formation surface, on which a latent image is formed on an image formation surface by exposure, the light-emitting device according to the aspect of the invention that exposes the image formation surface, and a developing device that attaches a developing agent (for example, a toner) to the latent image so as to form an apparent image. In the light-emitting device according to the aspect of the invention, irregularity of the light amount (gray-scale level) of each light-emitting element can be suppressed for a long period. Therefore, according to the image forming apparatus using the light-emitting device, uniform-quality images can be formed on recording mediums for a long period.
According to a specific aspect of the image forming apparatus, the first period may be a period where a developing device forms an apparent image from a latent image formed on an image carrier by light-emission of each light-emitting element in that period, and the second period may be a period, a gap between previous and next first periods, where the apparent image according to light-emission of each light-emitting element is not formed in that period. According to this configuration, since light-emission in the second period does not have an effect on the visual image to be formed in the first period, a desired image can be formed with high quality. Moreover, a configuration for causing the visual image (apparent image) according to light-emission by each light-emitting element not to be formed in the second period is arbitrarily selected. For example, a configuration for causing a developing agent not to be attached to a latent image formed on a photosensitive member by light-emission of each light-emitting element in the second period may be selected. Alternatively, a configuration for causing the latent image not to be formed on the photosensitive member by light-emission of each light-emitting element in the second period (for example, causing the photosensitive member not to be charged in the second period) may be selected.
Besides, the use of the light-emitting device according to the invention is not limited to exposure. For example, the light-emitting device according to the aspect of the invention can be used as display devices of various electronic apparatuses. Such an electronic apparatus includes, for example, a personal computer or a cellular phone. Further, the light-emitting device according to the aspect of the invention is suitably used as various illumination devices, such as a device that is disposed at the back of a liquid crystal device and illuminates the liquid crystal device (backlight) or a device that is mounted on an image reading apparatus, such as a scanner or the like, and irradiates light onto an original.
Another aspect of the invention is also specified as a circuit for driving a light-emitting device (a driving circuit 30 and a controller 40 of
Another aspect of the invention is also specified as a method of driving a light-emitting device. A method of driving a light-emitting device, which has a plurality of light-emitting elements each emitting light with a light amount according to a driving signal, includes acquiring first gray-scale data assigning a gray-scale value of each of the plurality of light-emitting elements, generating second gray-scale data from the acquired first gray-scale data for each light-emitting element such that, as the gray-scale value assigned by the first gray-scale data is large, the gray-scale value assigned by the second gray-scale data is made small, and causing each light-emitting element to emit light upon supply of a driving signal according to the acquired first gray-scale data in a first period, and causing each light-emitting element to emit light upon supply of a driving signal according to the generated second gray-scale data in a second period different from the first period. According to this driving method, the same effects as those in the light-emitting device according to the aspect of the invention can be obtained.
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
<A: Configuration of Light-Emitting Device>
The configuration of a light-emitting device according to an embodiment of the invention will be described. The light-emitting device is an exposure head that exposes a photosensitive member, such as a photosensitive drum or the like, and forms a latent image (electrostatic latent image) on a surface of the photosensitive member. In this embodiment, it is assumed that a latent image having pixels of vertical m row×horizontal n columns is formed (where m and n are natural numbers).
The head module 20 includes a light-emitting unit 72 and a driving circuit 30. In the light-emitting unit 22, n light-emitting elements E are linearly arranged along a main scanning direction. The light-emitting elements E correspond to n pixels constituting each row of the image. In this embodiment, each of the light-emitting elements E is an OLED element that has an anode and a cathode, and a light-emitting layer formed of an organic electroluminescent interposed between the anode and the cathode.
The driving circuit 30 causes the n light-emitting elements E to emit light according to the supply of driving signals X1 to Xn. The driving signal Xj that is supplied to the light-emitting element E of the j-th column (where j is an integer satisfying the condition 1≦j≦n) keeps a current value (hereinafter, referred to as ‘on current value Ion’) for causing the light-emitting element E over a time length according to a gray-scale value assigned to the light-emitting element E in a predetermined period (hereinafter, referred to as ‘unit period’). In the remaining period of the unit period, the driving signal Xj becomes zero. Moreover, the driving circuit 30 may have a plurality of IC chips each driving the predetermined number of light-emitting elements E or may have a single IC chip driving all the light-emitting elements E. Further, the driving circuit 30 may have thin film transistors. In this configuration, the light-emitting elements E and the driving circuit 30 are integrally incorporated into a surface of a substrate formed of an insulating material, such as glass or the like.
As shown in
In each of the unit period U1 of the first period P1, a degree of light-emission of each light-emitting element E varies according to an image content (a gray-scale value of each pixel). Therefore, if each light-emitting element E is allowed to emit light in only the first period P1, a degree of time-variant characteristic degradation varies for each light-emitting element E, and thus a variation in light amount (luminance) of each light-emitting element E occurs. In this embodiment, in order to prevent the variation, in the second period P2, each light-emitting element E is allowed to emit light at luminance having inverted high and low levels with respect to those in the first period P1. For example, the j-th light-emitting element E is allowed to emit light at high luminance in the i-th (where i is an integer satisfying the condition 1≦i≦m) unit period U1 of the first period P1, while the j-th light-emitting element E is allowed to emit light at low luminance in the i-th unit period U2 of the second period P2. According to this configuration, in the first period P1 and the second period P2 immediately after the first period P1, the degree of light-emission of each light-emitting element E (and a degree of degradation of each light-emitting element E according to light-emission) can be made uniform over n light-emitting elements E, regardless of the image content. Moreover, since the second period P2 is a period where the image is not output to the outside, light-emission of the light-emitting element E in the second period P2 does not nave effect on the image to be formed on the recording medium in the first period P1.
The image data G includes a plurality (‘m×n’) of first gray-scale data DGa that defines gray-scale values of the pixels of vertical m rows×horizontal n columns included in the image for one page. The first gray-scale data DGa is sequentially input to the controller 40 at a timing synchronized to the clock signal DCK0. The first gray-scale data DGa corresponding to one pixel is 4-bit digital data for defining the gray-scale value of the pixel to any one of 16 levels (‘0’ to ‘15’). Moreover, the number of bits of the gray-scale data (the first gray-scale data DGa or second gray-scale data DGb to be described below) is arbitrarily set. For example, the number of bits may be six or eight.
As shown in
The clock control unit 42 generates and outputs a clock signal DCK1 having a cycle according to the mode signal Smod from the clock signal DCK0. As shown in the portion (1) of
As shown in
The storage unit 44 of
The data processing unit 45 outputs n gray-scale data DG (DG[1] to DG[n]) for assigning the gray-scale values of the individual light-emitting elements E to the driving circuit 30 on the basis of the image data G stored in the storage unit 44 and the mode signal Smod to be supplied from the host device. The n gray-scale data DG (DG[1] to DG[n]) is sequentially output to the driving circuit 30 in synchronization with the clock signal DCK1 to be supplied from the clock control unit 42. The operation of the data processing unit 45 will be described in detail below.
The data processing unit 45 reads the n first gray-scale data DGa (DG[1] to DGa[n]) corresponding to each row among the image data G stored in the storage unit 44 for each unit period U (U1 or U2) of the first period P1 or the second period P2. For example, in each of the first period P1 and the second period P2, n first gray-scale data DGa (DGa[1] to DGa[n]) corresponding to the i-th row of the image in the i-th unit period U is read in parallel.
In the first period P1 where the mode signal Smod is kept at the high level, as shown in the portion (1) of
The second gray-scale data DGb[j] generated in the i-th unit period U2 of the second period P2 is data for assigning a gray-scale value having inverted high and low levels (density of a gray-scale level) with respect to the first gray-scale data DGa[j] of the pixel of the i-th row and the j-th column. That is, the data processing unit 45 generates the second gray-scale data DC-b[j] from the first gray-scale data DGa[j] such that the larger the gray-scale value of the first gray-scale data DGa[j] read from the storage unit 44 is, the smaller the gray-scale value of the second gray-scale data DGb[j] is (the smaller the gray-scale value of the first gray-scale data DGa[j] is, the smaller the gray-scale value of the second gray-scale data DGb[j] is). More specifically, the data processing unit 45 generates 4-bit data, which is obtained by inverting all the bits (4 bits) of the first gray-scale data DGa[j] as the second gray-scale data DGb[j]. For example, when the first gray-scale data DGa[j] is “0110” in binary notation (‘6’ in decimal notation), the data processing unit 45 generates “1001” obtained by inverting the individual bits (‘9’ in decimal notation) as the second gray-scale data DGb[j]. Therefore, like the first gray-scale data DGa[j], the second gray-scale data DGb[j] is four-bit data for assigning any one of 16 gray-scale values in total.
The timing control unit 47 of
As shown in the portions (1) and (2) of
The pulse width defining clock PCK is a clock signal for defining a timing at which a current value of the driving signal Xj is switched.
As shown in
The cycle tc2 of the clock signal DCK1 in the second period P2 is shorter than the cycle tc1 in the first period P1. Accordingly, as shown in the portions (1) and (2) of
The current value setting unit 48 determines the on current values Ion of the driving signals X1 to Xn according to the mode signal Smod. More specifically, the current value setting unit 48 outputs current value data DI for assigning the on current value Ia to the driving circuit 30 in the first period P1 where the mode signal Smod is in the high level, and outputs current value data DI for assigning the current value Ib higher than the on current value Ia to the driving circuit 30 in the second period P2 where the mode signal Smod is in the low level. Moreover, the detail relationship between the on current values Ion (Ia and Ib) will be described in details.
Next, the specific configuration of the driving circuit 30 shown in
The gray-scale data DG[1] to DG[n] are supplied in serial from the data processing unit 45 to the latch circuit 32. The latch circuit 32 samples the gray-scale data DG[j] and outputs the sampled gray-scale data at a timing at which the sampling signal Sj is changed to the active level. Therefore, the gray-scale data DG[1] to DG[n] are sequentially output to the latch circuit 33 for each one cycle of the clock signal DCK1. The gray-scale data DG[1] to DG[n] for one row that are sampled by the latch circuit 32 are simultaneously output from the latch circuit 33 at a timing at which the light-emission enable signal LE rises (see
At the back of the latch circuit 33, a pulse driving circuit 35 is disposed. The pulse driving circuit 35 includes n unit circuits S corresponding to the total number of the light-emitting elements E. The pulse width defining clock PCK is supplied from the clock control unit 42 to the individual unit circuits C. The j-th unit circuit C outputs a pulse driving signal PWj having a pulse width according to the gray-scale data DG[j] to be supplied from the latch circuit 33. That is, as shown in the portions (1) and (2) of
A current output circuit 37 of
With the above-described configuration, as shown in the lowermost side of the portion (1) of
As such, in this embodiment, the light-emitting elements E are driven on the basis of the first gray-scale data DGa in the first period P1, and are driven on the basis of the second gray-scale data DGb having the inverted high and low level with respect to the gray-scale value of the first gray-scale data DGa in the second period P2. According to this configuration, regardless of the content of the image data C (the first gray-scale data DGa), the sum of the degrees of light-emission (light-emission energy) over the first period P1 and the second period P2 can approximate to a predetermined value. Accordingly, the degrees of time-variant characteristic degradation are made uniform for the plurality of light-emitting elements E. Therefore, according to this embodiment, a variation of light amount of each light-emitting element E due to the difference in time-variant characteristic degradation can be suppressed.
In addition, in this embodiment, with driving according to the first gray-scale data DGa and driving according to the second gray-scale data DGb, the degrees of characteristic degradation of the light-emitting elements E are made uniform, and thus an accumulation value of the number of light-emission times of each light-emitting element E does not need to be held. Therefore, as compared with the configuration of Patent Document 1 or 2 where the accumulation value of the number of light-emission times for each light-emitting element E is held, a required storage capacity of the light-emitting device 10 is markedly reduced. For example, in this embodiment, the required storage capacity of the light-emitting device 10 is merely about ‘4 (bits)×m (rows)×n (columns)’. With the reduction in the storage capacity, the circuit size of the light-emitting device 10 can be reduced or manufacturing costs can be reduced.
Further, in this embodiment, the second period P2 for adjusting the degree of degradation of the light-emitting element E is disposed in an interval (a so-called paper interval) between the first periods P1 where the image according to exposure by the light-emitting device 10 is actually output. That is, even though the light-emitting elements E emit light in the second period P2, light-emission does not have a direct effect on an original use of an image forming apparatus. Besides, since the second period P2 is set to have the time length shorter than that of the first period P1, a ratio of a time at which the image forming apparatus can be effectively used for the original use can be increased, as compared with a configuration where the first period P1 and the second period P2 have the same time length. That is, effective image formation (printing at a high speed) can be implemented. In addition, even though the pulse width of the driving signal Xj in the second period P2 is set to have the time length shorter than that of the pulse width of the driving signal Xj in the first period P1, the on current value Ib of the driving signal Xj in the second period P2 is set to have the current value higher than the on current value Ia of the driving signal Xj in the first period P1. Therefore, the degrees of light-emission over the first period P1 and the second period P2 can be made uniform at high accuracy for each light-emitting element E.
Next, a specific method of selecting the current value Ia and the current value Ib will be described with reference to
The lifespan LT2 of the light-emitting element E when light-emission of the portion (b) of
First, as shown in a portion (a1) of
LT1=LT0×(Lb/La)−M (1)
In the equation (1), ‘M’ is a multiplier factor that is determined according to a material, a structure, or a manufacturing method of the light-emitting element E, for example, ‘2’ or ‘3’. As seen from the equation (1), the lifespan LT1 of the light-emitting element E is in inverse proportion to an M power of the intensity Lb. That is, a speed of characteristic degradation of the light-emitting element E is in proportion to the M power of the intensity Lb.
Next, as shown in the portion (b) of
LT2=LT1×(Wa/Wb) (2)
As seen from the equation (2), the lifespan LT2 of the light-emitting element E is in inverse proportion to the pulse width Wb. That is, the characteristic of the light-emitting element E is being degraded at a speed proportional to the pulse width. As seen from the equations (1) and (2), the state of a change in electrical or optical characteristic of the light-emitting element E (a characteristic degradation speed) in this embodiment varies between a case where the on current value Ion changes while the pulse width of the driving signal Xj is kept as it is (the equation (1)) and a case where the pulse width changes while the on current value Ion of the driving signal Xj is kept as it is (the equation (2)).
Here, in order to equalize the degree of degradation (lifespan) in the case of the portion (a) and the case of the portion (b) of
LT2=LT0 (3)
If the equation (3) is substituted with the equations (1) and (2), the following equation (4) is deduced.
(Lb/La)−M=(Wb/Wa) (4)
Now, it is assumed that the cycle tp2 of the pulse width defining clock PCK in the second period P2 becomes ‘1/u’ (u>1) times of the cycle tp1 of the pulse width defining clock PCK in the first period P1 (that is, the cycle tc2 of the clock signal DCK1 in the second period P2 becomes ‘1/u’ times of the cycle tc1 in the first period P1). In this case, the pulse width Wb of the driving signal Xj when the gray-scale value g0 in the second period P2 is assigned becomes ‘1/u’ times of the pulse width Wa of the driving signal Xj corresponding to the same gray-scale value g0 of the first period P1. That is, the following equation is established.
Wb=Wa/u (5)
If the equation (5) is substituted for the equation (4), the following equation (6) is deduced.
Lb/La=u1/M (6)
In this embodiment, the on current value Ia in the in the first period P1 and the on current value Ib in the second period P2 are determined such that the intensity La and the intensity Lb satisfy the equation (6). As seen from the above-described deduction processing, by selecting a relative ratio of the on current value Ia and the on current value Ib from the equation (6), the degrees of degradation of each light-emitting element E over the first period P1 and the second period P2 can be made uniform with high accuracy.
<B: Modifications>
Various modifications can be made from the above-described embodiment. Specific modifications are illustrated as follows. Moreover, the following modifications may be appropriately combined.
(1) First Modification
In the above-described embodiment, on an assumption that the second period P2 is set to the time length of 1/u times of the first period P1, a procedure for selecting the on current value Ion (Ia or Ib) has been described. In contrast, on an assumption that the on current value Ia of the first period P1 and the on current value Ib of the second period P2 have a predetermined ratio, the time lengths of the first period P1 and the second period P2 may be determined. For example, it is assumed that the on current value Ib in the second period P2 is set to be ‘v’, times of the on current value Ia in the first period P1 such that the intensity Lb in the second period P2 becomes ‘v’ (v>1) times of the intensity La in the first period P1. In this case, the following equation is established.
Lb=v×La (7)
If the equation (7) is substituted for the equation (4) the following equation (8) is deduced.
Wb/Wa=v−M (8)
Therefore, the relative ratio of the pulse width Wa in the first period P1 and the pulse width Wb in the second period P2 (that is, the ratio of the cycle tp1 of the pulse width defining clock PCK in the first period P1 and the cycle tp2 in the second period P2) is determined so as to satisfy the equation (8).
(2) Second Modification
In the first embodiment, the configuration that the cycle of the pulse width defining clock PCK varies in the first period P1 and the second period P2 by the change in cycle of the clock signal DCK1 is illustrated, but the cycle of the clock signal DCK1 is not necessarily changed. That is, the pulse width defining clock PCK having the cycle tp1 in the first period P1, and the pulse width defining clock PCK having the cycle tp2 in the second period P2 may be generated by the timing control unit 47. According to this configuration, what is necessary is that, only for a part for generating the pulse width defining clock of the timing control unit 47 and the pulse driving circuit 35 of the driving circuit 30, a processing is changed in the first period P1 and the second period P2. Therefore, the configuration of the light-emitting device 10 is simplified, as compared with the above-described embodiment where the cycle of the clock signal DCK1 is changed. Moreover, in this modification, the data processing unit 45 outputs the gray-scale data DGC[1] to DC[n] to the driving circuit 30 at a timing synchronized to the clock signal DCK0 (cycle tc1) supplied from the outside. However, in this configuration, the time length required for the output of the gray-scale data DG[1] to DG[n] synchronized to the clock signal DCK0 (at least the time length of ‘cycle tc1×n’) needs to be secured in a section from the rising edge of the start pulse SP to a timing at which the light-emission enable signal LE rises.
(3) Third Modification
In the above-described embodiment, the configuration that the current having the on current value Ion is supplied from the driving circuit 30 to the light-emitting element E is illustrate (a current-driven type), but the driving circuit 30 may apply a voltage to the light-emitting element E so as to cause the light-emitting element E to emit light (a voltage-driven type). Further, in the above-described configuration, the configuration that the gray-scale level (the sum of the light-emission amount in the unit period U) of the light-emitting element E is controlled by adjusting the pulse width of the driving signal Xj, but a method of controlling the gray-scale level of the light-emitting element E may be arbitrarily selected. For example, the gray-scale level of the light-emitting element E may be controlled by adjusting the level of the driving signal Xj (current value or voltage value). Therefore, for example, in each of the unit periods U1 of the first period P1, the driving signal Xj having a level according to the first gray-scale data DGa may be output from the driving circuit 30. Further, in each of the unit periods U2 of the second period P2, the driving signal Xj having a level according to the second gray-scale data DGb may be output from the driving circuit 30. According to this configuration, even though the pulse width of the driving signal Xj is not changed in the first period P1 and the second period P2, a difference of time-variant change in characteristic of each light-emitting element E can be suppressed.
(4) Fourth Modification
The division of the individual elements shown in
<C: Electronic Apparatus>
<C-1: Image Forming Apparatus>
Next, an image forming apparatus that is one example of an electronic apparatus according to the invention will be described with reference to
In the image forming apparatus, four light-emitting devices 10K, 10C, 10M, and 10Y having the same configuration are respectively disposed at positions facing image formation surfaces of four photosensitive drums (image carriers) 110K, 110C, 110M, and 110Y having the same configuration. The light-emitting devices 10K, 10C, 10M, 10Y are the light-emitting device 10 according to the above-described embodiment.
As shown in
In the periphery of the intermediate transfer belt 120, four photosensitive drums 110K, 110C, 110M, and 110Y having photosensitive layers on the circumferences are disposed at predetermined intervals. Symbols ‘K’, ‘C’, ‘M’, and ‘Y’ mean that the photosensitive drums 110K, 110C, 110M, and 110Y are respectively used to form apparent images of black, cyan, magenta, and yellow. The same is applied to other members. The photosensitive drums 110K, 110C, 110M, and 110Y rotate in synchronization with driving of the intermediate transfer belt 120.
Around each photosensitive drum 110 (K, C, M, or Y), a corona charger 111 (K, C, M, or Y), the light-emitting device 10 (K, C, M, or Y), and a developing device 114 (K, C, M, or Y) are disposed. The corona charger 111 (K, C, M, or Y) uniformly charges the image formation surface 110A (circumference) of the photosensitive drum 110 (K, C, M, or Y). The light-emitting device 10 (K, C, M, or Y) writes an electrostatic latent image on the charged image formation surface 110A of each photosensitive drum. In each light-emitting device 10 (K, C, M, or Y), a plurality of light-emitting elements E are arranged along a main bus (main scanning direction) of the photosensitive drum 110 (K, C, M, or Y). Writing of the electrostatic latent image is performed by irradiating light onto the photosensitive drum 110 (K, C, M, or Y) using the plurality of light-emitting elements E. The developing device 114 (K, C, M, or Y) attaches a toner serving as a developing agent to the electrostatic latent image so as to form an apparent image (that is, a visual image) on the photosensitive drum 110 (K, C, M, or Y).
The apparent images of black, cyan, magenta, and yellow formed by such monochrome apparent image formation steps for four colors are primarily transferred on the intermediate transfer belt 120 in sequence and overlap one another on the intermediate transfer belt 120. As a result, full color apparent images are formed. Four primary transfer corotrons (transfer devices) 112 (K, C, M, and Y) are disposed in the intermediate transfer belt 120. The primary corotrons 112 (K, C, M, and Y) are respectively disposed in the vicinities of the photosensitive drum 110 (K, C, M, and Y), and transfer the apparent images to the intermediate transfer belt 120 passing between the photosensitive drums and the primary transfer corotrons by electrostatically absorbing the apparent images from the photosensitive drums 110 (K, C, M, and Y).
A sheet 102 serving as a subject (recording medium), on which an image is finally formed, is fed from a paper feed cassette 101 by a pickup roller 103 one by one, and is then fed to a nip between the intermediate transfer belt 120 in contact with the driving roller 121 and the secondary transfer roller 126. The full color apparent images on the intermediate transfer belt 120 are collectively secondarily transferred to one surface of the sheet 102 by a secondary transfer roller 126, and then are fixed on the sheet 102 when the sheet 102 passes through a pair of fixing rollers 127 serving as a fixing device. Then, the sheet 102 is discharged on a discharge cassette, which is provided on an upper portion of the apparatus, by a pair of discharge rollers 128.
Next, another example of an image forming apparatus according to the invention will be described with reference to
The corona charger 163 uniformly charges the circumference of the photosensitive drum 110. The light-emitting device 10 writes an electrostatic latent image on a charged image formation surface 110A (circumference) of the photosensitive drum 110. In the light-emitting device 10, a plurality of light-emitting elements E are arranged along a main bus of the photosensitive drum 110 (in a main scanning direction). Writing of the electrostatic latent image is performed by causing light to be irradiated from the light-emitting elements E onto the photosensitive drum 110.
The developing unit 161 is a dream that has four developing devices 163Y, 163C, 163M, and 163K arranged at angular intervals of 90°. The developing unit 161 can rotate in a counterclockwise direction around a shaft 161a. The developing devices 163Y, 163C, 163M, 163K respectively supply toners of yellow, cyan, magenta, and black to the photosensitive drum 110, and form an apparent image (that is, visual image) on the photosensitive drum 110 by attaching the toners serving as developing agents to the electrostatic latent image.
An endless intermediate transfer belt 169 winds on a driving roller 170a, a driven roller 170b, a primary transfer roller 166, and a tension roller, and rotates around the vicinities of the rollers in a direction indicated by an arrow. The primary transfer roller 166 transfers the apparent image to the intermediate transfer belt 169 passing between the photosensitive drum 110 and the primary transfer roller 166 by electrostatically absorbing the apparent image from the photosensitive drum 110.
Specifically, with the first one rotation of the photosensitive drum 110, the electrostatic latent image for a yellow (Y) image is written by the light-emitting device 10, and the apparent image of the same color is formed by the developing device 163Y and is then transferred to the intermediate transfer belt 169. Further, with the next one rotation, the electrostatic latent image for a cyan (C) image is written by the light-emitting device 10, and the apparent image of the same color is formed by the developing device 163C and is then transferred to the intermediate transfer belt 169 so as to overlap the apparent image of yellow. During the photosensitive drum 110 rotates four times in such a manner, the apparent images of yellow, cyan, magenta, and black sequentially overlap on the intermediate transfer belt 169. As a result, the full color apparent images are formed on the transfer belt 169. When an image is formed on both surfaces of the sheet serving as a subject, on which the image is finally formed, the apparent images of the same color for both surfaces are transferred to the intermediate transfer belt 169. Then, the full color apparent images are formed on the intermediate transfer belt 169 by transferring the apparent images of the next color for both surfaces to the intermediate transfer belt 169.
In the image forming apparatus, a sheet feed path 174, through which the sheet passes, is provided. The sheets are fed from a discharge cassette 178 by a pickup roller 179 one by one, travels the sheet feed path 174 by a feed roller, and passes through a nip between an Intermediate transfer belt 169 in contact with a driving roller 170a and a secondary transfer roller 171. The secondary transfer roller 171 transfers the apparent images to one surface of the sheet by collectively and electrostatically absorbing the full color apparent images from the intermediate transfer belt 169. The secondary transfer roller 171 approaches or moves away from the intermediate transfer belt 169 by a clutch (not shown). Then, when the full color apparent images are transferred to the sheet, the secondary transfer roller 171 comes into contact with the intermediate transfer belt 169. Meanwhile, the secondary transfer roller 171 is separated from the secondary transfer roller 171 when the apparent images overlap on the intermediate transfer belt 169.
The sheet on which the images are transferred in such a manner is fed to a fixing device 172, and the apparent images on the sheet are fixed when the sheet passes through a heating roller 172a and a pressing roller 172b of the fixing device 172. After fixing, the sheet is pulled between a pair of discharge rollers 176 and moves in a direction of an arrow F. When double printing, most of the sheet passes through the pair of discharge rollers 176, and then is introduced into a two-sided printing feed path 175, as indicated by an arrow G, by the reverse rotation of the pair of discharge rollers 176. Next, the apparent images are transferred to the other surface of the sheet by the secondary transfer roller 171. After fixing is performed by the fixing device 172 again, the sheet is discharged by the pair of discharge rollers 176.
The image forming apparatus shown in
<C-2: Others>
In the above description, the light-emitting device that is used as an exposure head has been illustrated, but the use of the light-emitting device of the invention is not limited to exposure of a photosensitive member. For example, the light-emitting device of the invention is used in an image reading apparatus, such as a scanner or the like, as a linear optical head (an illumination device) that irradiates light on a subject to be read, such as an original or the like. Such an image reading apparatus includes a scanner, a reading part of a copy machine or a facsimile machine, a bar code reader, or a two-dimensional image code reader that reads a two-dimensional image code, such as a QR code (Registered Trademark). Further, a light-emitting device that has a plurality of light-emitting elements arranged in a planar shape is used as a backlight that is disposed on the back side of a liquid crystal panel.
As a display device for displaying an image, the light-emitting device according to the embodiment of the invention is used. In this display device, a plurality of light-emitting elements E are arranged in a matrix shape over a row direction and a column direction. Then, a scanning line driving circuit selects each row for each unit period (horizontal scanning period), and a driving signal Xj is supplied to the individual light-emitting elements E of the selected row from the driving circuit 30. An electronic apparatus in which the light-emitting device according to the embodiment of the invention is used to display the images includes, for example, a portable personal computer, a cellular phone, a personal digital assistant (PDA), a digital still camera, a television, a video camera, a car navigation device, a pager, an electronic organizer, an electronic paper, an electronic calculator, a word processor, a workstation, a video phone, a POS terminal, a printer, a scanner, a copy machine, a video player, an apparatus having a touch panel.
The entire disclosure of Japanese Patent Application No. 2005-328487, filed Nov. 14, 2005 is expressly incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-328487 | Nov 2005 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5999150 | Nighan et al. | Dec 1999 | A |
| Number | Date | Country |
|---|---|---|
| A-03-120059 | May 1991 | JP |
| A-2001-042458 | Feb 2001 | JP |
| 2001-284044 | Oct 2001 | JP |
| A 2002-361924 | Dec 2002 | JP |
| A 2003-334990 | Nov 2003 | JP |
| A-2003-341124 | Dec 2003 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20070109231 A1 | May 2007 | US |
